The present invention relates to a fixing device that fixes an unfixed toner image formed on a sheet medium. The fixing device comprises: a heating roller; a pressing roller that is pressed against the heating roller; a belt that moves by being wound around an outer periphery of the pressing roller and nipped between the pressing roller and the heating roller; and a belt stretcher over which the belt is stretched. The present invention also relates to an image forming apparatus incorporating such a fixing device.
An image forming apparatus such as a copying machine, a printer and a facsimile machine incorporates a heating roller type fixing device in which an unfixed toner on a recording medium is subjected to a press contact with heat to thereby fix the unfixed toner on the recording medium. Such a fixing device comprises: a rotatable heating roller coated with an elastic body on the surface thereof and housing a heat source therein; a belt stretched over plural supporting rollers, and a pressing member for forming a nip region by causing the belt to wind around the heating roller by a predetermined angle while causing the elastic body on the surface of the heating roller to undergo deformation by locally applying a larger pressure to the exit of the nip region than the other portions, thereby making it easier for a sheet medium to be discharged from the nip portion. Such a configuration is disclosed in Japanese Patent No. 3084692.
In the fixing device, the surface of the heating roller has been deformed in advance due to the presence of the pressing member, and the deformation on the surface is relieved momentarily while toner is in contact with the surface of the heating roller. For this reason, when a sheet medium is discharged from the nip portion, an contact force between the toner and the heating roller is reduced in inhibiting the sheet medium from winding around the heating roller, so that even a recording sheet of paper that is not very firm can be readily separated at the exit of the belt nip. This device thus eliminates the need for a separating claw member.
However, plural supporting rollers and the rotation bearings are required. Further, not only the peripheral length of the belt is extended, but also the fixing device becomes complex with an increase in both size and cost. The structure such that makes the fixing device complex with an increase in size and cost naturally makes the image forming apparatus equipped with such a fixing device complex with an increase in size and cost.
Moreover, the belt is heated at the nip portion with the rotatable heating roller housing the heat source; however, with the configuration in which the belt is stretched over plural rollers and its peripheral length is inevitably extended, heat energy is absorbed by plural supporting rollers when it moves along a predetermined path. In addition, natural heat release is increased with the peripheral length. This extends a time needed to reach a predetermined temperature, which in turn extends a so-called warm-up time needed for the fixing operation to be enabled since the apparatus has been activated. Hence, this configuration is not preferable.
Further, the configuration, in which deformation of the elastic layer on the heating roller is caused by winding the belt around the heating roller by an angle needed to form the nip and by applying a larger pressure locally at the exit of the nip portion than the other portions, is suitable in inhibiting a sheet medium from winding around the heating roller; however, the sheet medium discharged along the deformation of the elastic layer is distorted, by being curled along the deformation or wrinkled as a result of a local high pressure.
Japanese Patent Publication No. 6-40235B discloses a fixing device in which an unfixed toner image is fixed on a recording medium by passing through a nip region formed between roller members which are in press contact with each other. In this device, the rotating speed of the roller members can be set to either a first speed or a second speed in accordance with the characteristic of the recording medium.
However, since the heat capacity of the roller members are large, the warm-up time is extended. Moreover, the recording medium passing through the nip region formed by deforming the roller members with a pressure undergoes distortion, such as curls and wrinkles, because an applied stress induced by this pressure is large.
Japanese Patent Publication No. 8-262903A discloses a fixing device in which an endless belt stretched over a heating roller, which is coated with an elastic body on the surface thereof and housing a heat source therein, is run while being in contact with the heating roller, and a non-rotatable pressing pad, provided on the inner side of the endless belt to press the endless belt against the heating roller, not only forms a nip portion, but also causes the elastic layer on the surface of the heating roller to undergo deformation. A sheet medium bearing an unfixed toner image is forced to pass through a space between the heating roller and the endless belt for the toner to be fixed on the sheet medium. Since the pressing pad is not rotated, heat transmitted from the heating roller is hardly dissipated. Therefore it has an advantage that a quantity of heat absorbed from the heating roller is small.
Nevertheless, the warm-up time is extended because heat is transmitted to the pressing pad from the heating roller via the endless belt during the warm-up operation. In addition, three or more rollers are necessary to circulate the belt, which raises another problem that the device is increased in size.